They didn't let DD "slip through their fingers"; they gave him up to Marvel Studios in a calculated move to protect the FF franchise (specifically, Galactus and Silver Surfer). Thought you already knew that.

Here's the thing, though: Letting the Daredevil rights lapse was not a "calculated move to protect the FF franchise" because the one was not directly related to the other. By all accounts, Fox had a director for DD and a script but was unwilling or unable to finance the film on its own. (The notorious Fox cheapness at work.) As Deadline related, Fox went begging to Marvel/Disney for financial backing and an extension of its rights and was denied. The studio strung the director along until the very last minute, then decided to allow its rights to lapse rather than make the film. All of that occurred independent of any deal for the rights to Galactus and the Surfer.

It was only when Fox was at the end of its rope with the DD rights that Marvel offered them a deal for an extension in exchange for use of Galactus & the Surfer. This was after seven years of foot-dragging and nonsense from Fox, which could still have started filming on the movie up until the rights lapsed. You can spin that as Fox grandly sacrificing DD for the FF rights if you like, but the plain fact is that they were going to lose DD anyway because of their own mistakes and stinginess. Marvel simply offered them an out, which they declined to take.